As a consequence of rapid industrial growth, environmental disruption and disease have become more and more of a concern. Especially in recent years, the threat of SARS, Ebola and bird flu have raised awareness of the need for cleanliness and personal hygiene. As touch technologies proliferate, consumers are becoming increasingly aware of the possible existence of bacteria on mobile devices, particularly as touch-enabled surfaces are increasingly shared at home, work, and elsewhere. Therefore, there is an urgent need to develop effective and low cost cover glass that has antimicrobial properties.
Silver has long been known for its excellent antimicrobial properties; however, silver is relatively expensive and consequently cannot be fully utilized in industrial glass production. Most conventional antimicrobial glass has an antimicrobial layer of silver on the glass surface. Several methods are used to form this layer such as by adding silver to the raw materials for forming the glass, using silver salt spray pyrolysis, adding silver to the ion-exchange bath, coating the glass with silver, vacuum sputtering with silver and sol-gel processes for forming silver doped hybrid silicon dioxide transparent thin films from solutions that include silver nitrate and tetraethyl orthosilicate. Among such methods, adding silver to the ion-exchange bath is the most common and is the most likely technique to be used for mass production of glass having antimicrobial properties. Conventional ion exchange processes are used to chemically strengthen glass substrates and typically involve placing the glass in a molten salt containing ions having a larger ionic radius than ions present in the glass, such that the smaller ions present in the glass are replaced by larger ions from the molten salt solution. Typically, potassium ions in the molten salt replace smaller sodium ions present in the glass. The replacement of the smaller sodium ions present in the glass by larger potassium ions from the heated solution results in the formation of a compressive stress layer on both surfaces of the glass and a central tension zone sandwiched between the compressive stress layers. The tensile stress (“CT”) of the central tension zone (typically expressed in megapascals (MPa)) is related to the compressive stress (“CS”) of the compressive stress layer (also typically expressed in megapascals), and the depth of the compressive stress layer (“DOL”) by the following equation:CT=CS×DOL/(t−2DOL)
where t is the thickness of the glass.
Conventional ion exchange methods for making glass having antimicrobial properties include a one-step method in which silver is added to the conventional ion exchange bath. Glass produced by the one-step ion exchange method, however, has certain disadvantages such as silver colloidization which lowers the transmittance of visible light, low antimicrobial efficacy due to a low concentration of silver on the surface of the glass, and significant amounts of silver which reside in a deep ion exchange layer of the glass that has no effect on the antimicrobial properties of the glass.
Glass that simply incorporates silver as a component of the batch materials used to form the ion-exchangeable glass also has shortcomings. Specifically, the glass that results from such batch materials will have a low concentration of silver on the glass surface and will therefore have poor antimicrobial properties. If attempts are made to overcome this problem by including a high concentration of silver in the batch materials, the glass that results will have a visible yellow color and will have reduced antimicrobial properties due to silver colloidization caused by the high temperature ion exchange process which will lead to a decrease in the glass transmittance.